Thermal energy

hello. Welcome to my physic blog 2009. This blog belongs to Goh Wan Zhen of 3A1.

INTRODUCTION to Thermal Energy:

What causes transfer of thermal energy?

difference in temperature

Example:
Why do you feel cold when you touch ice water?

Human body temperature (approx 37°C) is warmer then ice water( approx 0°C). Hence, when a human is in contact with ice water, he will feel cold. Thermal energy form the human had been transferred to the ice water. Hence, the human will feel cold.

However, Thermal Energy will not be transferred when there is thermal equilibrium (no difference in temperature). Thus, there will be no gain or loss in Thermal Energy. (Refer to picture)

Ways Thermal Energy Is Transferred
Thermal energy is transferred by 3 processes

• conduction


• convection


• radiation

This is an example about these 3 processes:

Conduction

Definition: conduction is the process of thermal energy transfer without any flow of the material medium.
Takes place in all solids
For non-metals
1.Transfer of thermal heat by atomic/molecular vibrations

• When particles (atoms and molecules) are supplied heat at one end of the solid, particles at the hot end vibrate vigorously.
  
• Vibrating particles collides with neighboring particles, transferring kinetic energy to them.

For metals only (faster thermal heat transfer):
1.Transfer of thermal heat by atomic/molecular vibrations
2.Free electron diffusion

Free electron diffusion:

• Free electrons in heated metal gains kinetic energy and moves faster.
  
• Fast moving electrons would diffuse from the hotter parts of the metal to cooler parts of the metal, transferring kinetic energy to them.

• Thus, it takes lesser time for thermal energy to transfer in metals (good conductor of heat) than non-metals, Eg. wood (poor conductor of heat)

For liquids and gases (process inefficient):

• molecules of liquids and gases are spaced further apart than the molecules of solids

1.solids(closest)
2.liquids
3.gases(furthest)

• Therefore, collisions of particles in liquids and gases are less frequent than in solids.


• Less collisions, lower transfer rate.

Thus, solid is the best conductor of heat, followed by liquids and gases.

video of conduction:

This video shows the conduction in metals:

Think: why did the balloon that is filled with water did not explode?

Interesting fact of conduction:

Take note of the copper and iron part. The paper covering the copper did not burn while the paper covering the iron is burned. (copper is a better conductor than iron. Thermal energy had been transferred from the paper to the copper. Hence, the paper did not burn:))

Common applications of conduction:
Uses of good conductors:

• Cooking utensils where direct heating is involved(saucepans, kettles)
  
• Heat exchangers

Uses of bad conductors:

• Handles and appliances of utensils (to pick up utensils without scalding)
  
• Table mats (usually made of cork to prevent hot kitchenware form damaging the table tops)
  
• Sawdust (to cover ice blocks due to its good insulating property)
  
• Wooden ladles (stirring/ scooping food)
  
• Woolen clothes (keep people warm)
  
• Fibreglass, Felt, Expanded polystyrene foam (traps large amount of air that are employed as insulators for ice boxes etc./ Double -glazed windows have air trapped between the two glass, which reduces thermal transfer through the windows )

Convection
Definition: convection is the transfer of thermal energy by means of currents in a fluid (liquid/ gas).

Convection current is set up due to a difference in density. However, convection current only occurs in fluids such as gases and liquids, not solid.

A video of convection currents:





Notice how the liquid rises and falls. this movement is called convection current.
1.Water at the bottom of the beaker is heated and expands first

2.expanded water has a lower density than the surrounding water ( water that has not been heated and expanded)

3.cooler water become denser than the heated water

4.the cooler water sinks

5. this movement is called convection current.

*This theory/concept applies to gases as well

Convection:

• Involves bulk movement of the fluids, which carries thermal energy.
  
• Solids cannot set up convection because thermal energy in solid is transferred from one particle to another by vibrations, without any bulk movements of the particles itself.

Thinking questions: why do you feel hot when your hand is above a small fire?

Common applications of convection:

Electric kettles:

Air conditioners and refrigerators:
Ever wondered why air conditioners and freezing units in the refrigerators are always placed at the top?
It is to allow convection current to set up. Freezing units cool the surrounding air, resulting it to become denser than the surrounding air. When there is a difference in density, the cooler air sinks and the warmer air rises.


Sea breeze and Land breeze:

Sea breeze(day):
During the day, the Sun heats the land much faster than the sea (water is a poor conductor of heat). Hence, the heated air on the land expands and rises, while the cool air above the sea moves towards the land. This natural convection is called sea breeze.

Land breeze(night):
At night, the Sun cools the land much faster than the sea. hence, the warm air from the sea will rise and the cool air from the land will move towards the sea to replace the warm air. This convection is called land breeze.

Know more:
Know how thermal energy transfer from convection to conduction:


Radiation
Definition: radiation is the continual emission of infrared waves from the surface of all bodies, transmitted without the aid of a medium.

-Emitted from the surface of all bodies

-Does not require a medium

-Can take place in a vacuum

-Continual emission of thermal energy in the form of infrared waves

Sun: emits electromagnetic waves (infrared waves)

-Infrared waves has radiant heat (thermal energy)

-Thermal energy reaches the Earth by radiation

-Impossible by conduction or convection because of the vacuum (*conduction and convection
requires a medium to take place and convection can only be set up in fluids (gases/ liquids))



Factors that affect the rate of radiation:
Colour and texture
Dull and black surfaces are better absorbers and emitters of infrared radiation
Shiny and white surfaces are poorer absorbers and emitters of infrared radiation

Surface temperature
Higher temperature of the surface of the object relative to the surrounding temperature, the higher the rate of infrared radiation.

Surface area:
Do all objects made of the same material and has the same mass radiates heat at the same rate?
NO!!! An object with a larger surface area will radiate heat faster than another object with the same mass and material.

Common applications of radiation:
-Teapots
Shiny teapots are poor absorbers and poor emitters of radiation. Hence, it would maintain the temperature of the liquids.

-Greenhouses
It is very useful for plants to grow in cold climates.


-Vacuum flasks
To keep the liquid warm.


Questions:

1. Double glazing improves the heat insulation of the houses because________

a) Glass of double thickness do not conduct heat

b) Radiation does not pass through two sheets of glass

c) The trapped air between the two glasses is a bad conductor of heat

d) Convection currents between the two sheets are restricted

ANS: (C)

2. A solar heater uses heat energy from the Sun to heat water. The panels of the heater are painted black. Why is this so?

a) To improve absorption of infra-red radiation

b) To improve emissions of infra-red radiation

c)To improve the conducting properties of the panel

d)To reduce the convection currents

ANS: (A)

3. Which is a useful application of thermal expansion?

a) Stack telephone wires

b) Flexible joints in steam pipes

c) Gaps in concrete bridges

d) Bimetallic thermostat
ANS: (D)

4. Which of the following is the best thermal insulator?

a) Still air

b) Cork

c) Fibre glass

d) Plastic

ANS: (A)

5. Heat transfer from a hot flame to a water in a saucepan takes place by_____

a) Conduction and convection

b) Conduction and radiation

c) Convection and radiation

d) Conduction, convection and radiation

ANS: (D)

These are the sources where i got the pictures and videos from:

Sun radiation pic:
http://coolexcooling.com/wp-content/uploads/2008/06/imagedisplayim.jpg

Land and sea breeze pic:
http://www.free-online-private-pilot-ground-school.com/


Radiation, conduction and convection:
http://www.youtube.com/watch?v=VzE_IPedujc


Conduction current:
http://www.youtube.com/watch?v=1r_AJ7qUkts

Conduction Experiment (with Balloons):
http://www.youtube.com/watch?v=ZUNj4hkA4iU

Jillus miller science experiments:
http://www.youtube.com/watch?v=cMmREKOhIV8

Convection current:
http://www.youtube.com/watch?v=9FcfMGDdpCY&eurl=http%3A%2F%2Fgohwanzhen%2Dphyiscproject2009%2Eblogspot%2Ecom%2F&feature=player_embedded

Conduction and Convection:
http://www.youtube.com/watch?v=y8yT6h-JSIE